1. Field of the Invention
The present invention relates to an improvement in a throwaway cutting insert used to cut workpieces made of cast iron and similar materials.
2. Prior Art
A throwaway cutting insert having the shape of a polygonal plate, is generally secured on an insert support of a holder. The cutting edge of the cutting insert cuts the surface of a workpiece made of cast iron, steel, etc. to the desired shape, as the workpiece is rotated. When cutting a workpiece made of steel, a chip produced in the shape of a wire or rod during cutting may damage the workpiece surface and injure the hands of the operator. To solve this problem, the so-called breaker is provided near the edge of the cutting insert so that such a continuous chip is minutely broken or wound in the form of a spring when the chip is produced. When cutting a workpiece made of cast iron, however, such a chip is broken into pieces at moment the chip is produced. This can be explained by the characteristic of cast iron. Therefore, it is not necessary to provide such a breaker when cutting a workpiece made of cast iron. Furthermore, a cutting insert with no breaker is superior in the strength of the .edge. Because of these reasons, a cutting insert with no breaker is used preferably to cut workpieces made of cast iron. Such a cutting insert is generally made of cermet or cemented carbide. A coating insert is also used. A cutting insert with no breaker is generally case-hardened. The case-hardened cutting insert may often warp as shown in FIG. 9 during case hardening. If a warped cutting insert c is secured on a holder, the cutting insert makes contact with the supporting surface h.sub.1 of the holder h at the very small contact areas of the edge portions c.sub.1. Stress concentration occurs and the edge portions may be broken. If the cutting insert c is installed upside down as shown FIG. 9 (b), the cutting insert cannot stably make contact with the supporting surface h.sub.1 and may be broken due to impact during cutting. To solve these problems, only the rake surfaces c.sub.2 are made flat by grinding because these surfaces make contact with the supporting surface h.sub.1 when the cutting insert is installed as shown in FIGS. 9 (a) and 9 (b). More specifically, the portions shaded in FIG. 9 (c) are ground to obtain flat surfaces. However, this grinding increases the cost of the cutting insert and improvements in the cutting insert are thus desired.